Device for producing x-rays



April 27, 1954 c. H. BACHMAN DEVICE FOR PRODUCING X-RAYS Filed May 26,1951 l lll I Il l IV I R m m w.

Patente-d Apr. 27, 1954 UNITED STATES PATENT OFFICE 'DEVICE FORPRODUCING X-RAYS Charles H. Bachman, J amesville, N. Y. Application May26, 1951, Serial No. 228,402

(Cl. 313-v55) 3 Claims. l

vThis invention relates to a device for the production of X-rays having`preferred directional characteristics and to apparatus including thedevice.

Conventional X-ray tubes comprise a source of electrons and means offocusing these electrons to a small point on -a target which is theX-ray source. X-rays radiate in all directions from the focal spot onthe target. Disadvantages of such tubes include -the dilliculty offocusing the electrons to as 'lne a spot as possible, the problem ofdissipating the heat generated at the focal spot, and the fact that, dueto the divergent nature of the radiation from this focal spot, the X-rayintensity diminishes rapidly as the distance from the target isincreased.

It is the object of this invention to avoid these dilculties byproviding a device which generates X-rays without the necessity forfocusing the electron beam; which makes use of an exceptionally largetarget area, thus minimizing heating; and which by virtue of geometricalconsiderations in its design allows preferential selection of theradiation from the target so that the resulting Xray beam is divergent,parallel, or convergent to a point, as desired. Another `object is toprovide apparatus for using the device in suitable thereapeuticapplications.

Basically, the device comprises an X-ray tube 'having a conical shapedAtarget colinear with an axially arranged electron source, andassociated shields or collimators to aid in selecting the desiredportion of the radiation. A feature of the device is that the targetperforms part vof the collimating function. Another feature is that asthe axial length of the conical target is sincreased, the X-ray outputmay be increased proportionally with the increase in area.

In describing this invention, Ireference is had to the accompanyingdrawings in which like characters designate corresponding parts in allthe views.

Figure l is a diagrammatic :view of the target having a conical bore orpassage, and the filament or source of electrons in the passage.

Figure 2v is a diagrammatic View showing the device in longitudinalsection.

Figures 3a and 3b are longitudinal sectional views of slightly modiedforms of the target.

Figure 4 is a View, similar to Figure 2, of a `modified form of thedevice.

Figure 1 is a section showing a target l with aconical bore coaxialelectron source or filament 2. The'target is 'at a positive potentialwith respect to the filament. The lament,

when heated, emits electrons which are Vdrawn radially to all portionsof the wall of `the bore of the target. The shaded areas show thesectioned distribution pattern of X-radiation which is generated at thetarget surface. The doubly shaded areas 3a, 3b, indicate regions of veryhigh radiation density since these regions receive contributions fromall target areas. It should be noted that the boundaries of both theconverging and diverging regions are set by the shielding or collimatingaction of the target itself. Also should be noted the region beyond thecross-over point or cone apex 4a which is devoid of any radiation as aresult of the self collimation of the target cone. Also, note that eachpoint in the doubly shaded lregions receives contributi-ons from alltarget areas so the apex, lor cross-over point, receives 'itscontributions from rays which travel at grazing incidence to the surfaceof the cone.

It will be seen from Figure 1, that the shape of the distributionpattern of the doubly shaded areas and in particular the position of theapex, or cross-over point, is independent -of the axial length of theconical target. Furthermore, since all portions of the conical borecontribute to these regions, the actual radiation density may be variedwithin wide limits without yexcess heating of `the target by merelyvarying the axial length of the cone.

Figure 2 shows, in section, one embodiment of the device. The conicaltarget I enclosed in the vacuum chamber l is arranged to be concentricwith the electron emitting lament 2, whose lead wires enter the vacuumchamber through the insulator In rthis embodiment, a collimating plug orshield 5 is so placed that the only radiation ensuing from the tube isthat making small angles with the target wall. With this combination,the X-ray beam consists of a hollow conical sheath 9 which converges tothe lcross-over point ila, after which it expands as a similar 'hollowcone. Note that the intensity at the cross-over has 'been unchanged bythe addition of the collimating plug. It still receives contributionsfrom the entire target. The effect of the collimating `plug is to removethe inner portions defining the inner surface of the converging cone andthe outer surface of the diverging cone. This `plug may be used interiorto, or exterior to, the vacuum chamber. Without it, the X-ray intensityis substantially constant along the yaxis within the region 3a of"Figur-e' 1, but drops to zero at `points on the axis past lla. The selfcoll-imation of the target denes the outer surface of the convergingcone and the inner surface of the diverging cone.

The size of the cross-over region is determined by the Width of theannular collimating passage formed by the collimating plug 5 and thetarget I. Note that the radiation density at the crossover is muchgreater than at any other region in the sheath, either before or aftercross-over, since all the radiation in the sheath passes through thisregion. Therapeutically, this embodiment is capable of delivering a muchgreater useful radiation dose than other existing X-ray tubes,

As is seen from Figures 1 and 2, the cross-over is determined by theself collimating properties of the target, this limitation beingaccomplished by absorption of the X-radiation in the target. The smallerthe angle made with the target Wall by the grazing rays, the greater thechance that they will be absorbed by irregularities on the targetsurface. This effect increases with axial length of the target and makesitself evident as a reduction in the intensity at the cross-over.

Figures 3a and 3b show means for minimizing this effect for any givenwidth of collimating groove and length of target cone.

In Figure 3a, the target cone is shown made up of a series of conesarranged togive a steplike effect, the total rise of the steps beinglimited to the maximum allowable Width of the radia tion sheath asdetermined by the collimating plug and the target. The effect of this isto minimize the self absorption of X-radiation by the target. Thus,radiation from step b is not absorbed by step a. In general, the greaternumber of steps, the less self-absorption. If the number of stepsbecomes very great, the effect approaches that of a smooth curve, asshown in Figure 3b. This condition is achieved only at the loss of someof the self collimating properties hitherto described.

Although the practical embodiment shown in Figure 2 makes use of theradiation issuing from the convergent end of the conical target, usefuldirectional effects can also be obtained with a tube provided with aWindow to allow the radiation to emerge from the divergent end of thetarget.

Referring to Figure 1, every pointV in the doubly cross hatched region3b diverging from the conical target and indeed self collimated by thetarget contains radiation contributions from all portions of the targetl.

Figure 4 shows an embodiment utilizing portions of this radiation insuch a way as to obtain a beam of X-rays in the form of a substantiallysolid cone convergent to a point of crossover and diverging again afterthis cross-over. It will be seen that the X-ray tube structure isidentical with that of Figure 2 except that the collimating plug hasbeen omitted, and the conical target l has been reversed to present itslarger or open end to the tube Window 6. Thin walled metal tubing I0 maybe arranged in a bundle exterior but not limited to the exterior of thetube as shown. If the tubes are parallel in the bundle, the radiation iscollimated to be parallel. Likewise, they can be arranged to produce acollimated divergent beam or a -collimated convergent beam, as is shownin Figure 4. Such collimators may be used With the radiation from eitherthe converging or diverging end of the conical target for a range ofconvergence or divergen-ce angles determined by the self collimatingproperties of the target itself.

Either the embodiment of Figure 1, or of Fig-` Ll (l ure 4, hasimportant therapeutic value. For example, in the treatment of deepseated tumor, the position of the patient is adjusted so that thecross-over point 4a or 4b falls upon the tumor. In such instance, theradiation density at the tumor is greater than that in the tissue eitherbefore or after it. Thus, by the geometry of its X-ray beam, this tubeoffers an improvement over the conventional type X-ray tube usingparallel or diverging radiation since in those tubes the radiationdensity constantly diminishes along the beam.

What I claim is:

l. In an X-ray generating device, an X-ray tube having a cathodefilament and circuits therefor, said device being characterized by atarget having an internal truncated conical bore, the lament beinglocated within the bore and extending lengthwise of the axis of thebore, and means at the smaller end of the conical bore to conne theX-rays generated by the target in a hollow conical path having a focalarea located in the axis of the conical bore beyond the smaller end ofthe bore of the target.

2. In an X-ray generating device, an X-ray tube having a cathodefilament and circuits therefor, said device being characterized by atarget having an internal truncated conical bore, the filament beinglocated Within the bore and extending lengthwise of the axis of thebore, and means at the smaller end of the conical bore to conne theX-rays generated by the target in a hollow conical path having a focalarea located in the axis of the conical bore beyond the smaller end ofthe bore of the target, said means comprising a shield located Withinthe smaller end of the conical bore and having its periphery spaced fromthe opposing walls of the bore providing an annular outlet passage forthe X-rays generated in the target.

3. In an X-ray generating device, an X-ray tube having a cathodefilament and circuits therefor, said. device being characterized by atarget having an internal truncated conical bore, the lament beinglocated Within the bore and extending lengthwise of the axis of thebore, and means at the smaller end of the conical bore to confine theX-rays generated by the target in a hollow conical path having a focalarea located in the axis of the conical bore beyond the smaller end ofthe bore of the target, said means comprising a shield located withinthe smaller end of the conical bore and having its periphery spaced fromthe opposing walls of the bore providing an annular outlet passage forthe X-rays generated in the target, the conical bore being provided witha series of successive annular steps on the wall of the bore of thetarget with the risers of the steps facing toward the annular outlet.

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